John Gill Commentary Mark 6:50

"for they all saw him, and were troubled. But he straightway spake with them, and saith unto them, Be of good cheer: it is I; be not afraid." — Mark 6:50 (ASV)

For they all saw him, and were troubled
Had it been only seen by one, it might have been thought a fancy, and the effect of mere imagination; but as every one saw it, it was out of all doubt that so it was, and which gave them the greater concern:

and immediately he talked with them ;
as soon as they saw him, "that very moment", as the Syriac renders it; that so by hearing his voice their fears might be allayed:

and says unto them, be of good cheer, it is I, be not afraid ;
(See Gill on Matthew 14:27).